With the increase in the usage of digital technology in the TV/video industry, different digital video signal formats have emerged. These formats specify different attributes of a video signal.
With the advent of HDTV, the number of formats has grown further. In addition, because of differing requirements, any HDTV format will be quite different from the "old" NTSC/PAL/SECAM formats. The concurrent existence of multiple formats makes it desirable to convert from one signal format to another to facilitate use of newer equipment with the previously installed NTSC/PAL/SECAM format base.
An international format has been adopted for digital representation of a video component signal whose characteristics are described generally in Table 1 below.
TABLE 1 ______________________________________ Encoding Parameters for Digital Component Signals ______________________________________ Coded Signal Y = 0.299R + 0.587G + 0.114B C.sub.R = 0.713 (R-Y) = 0.500R - 0.419G - 0.081B C.sub.B = 0.564 (B-Y) = 0.500B - 0.169R - 0.331G Number of samples per line: Total Active luminance (Y) 858 720 each color difference (C.sub.R ' C.sub.B) 429 360 total number of samples 1716 1440 Sampling Structure Orthogonal: C.sub.r and C.sub.b samples are co-sited with odd (1st, 3rd, 5th) Y samples in each line. Sampling frequency: 13.5 MHz nominal luminance (Y) each color-difference signal 6.75 MHz nominal (13.5 MHz (C.sub.R ' C.sub.B) total) Correspondence between video signal levels and quantization levels: 8 bit system luminance signal (Y) 220 quantization levels with the black level at level 16 and the peak white level at level 235. each color-difference signal 225 quantization levels (C.sub.R ' C.sub.B) symmetrically distributed about the zero signal level 128. ______________________________________
The relationship between video signals in the digital and analog domain for 525-line systems for the digital component system can be summarized by noting that of the 1716 sample values generated over 1716 clock intervals:
(a) 1440 multiplexed luminance and chrominance values are transmitted during each active line, one per clock interval; PA0 (b) 8 of the remaining 276 clock intervals are used to transmit synchronizing information [as follows in (e) and (f)], the other 268 interface clock intervals may be used to carry ancillary information; PA0 (c) the first of these 1716 interface clock intervals is designated line word 0 for the purpose of reference only (the 1716 sample words per total line are therefore numbered 0 through 1715). PA0 (d) intervals through 1439, inclusive, contain video data--the interface clock intervals occurring during digital blanking are designated 1440 through 1715; PA0 (e) intervals 1440 through 1443 are reserved for the end-of-active-video (EAV) timing reference; PA0 (f) intervals 1712 through 1715 are reserved for the start of-active-video (SAV) timing reference; (g) the half amplitude point of the leading (falling) edge of the analog horizontal sync signal is coincident with a sample point which would be conveyed by word 1473 if carried across the interface. PA0 1) "Characteristics of Systems for Monochrome and Colour Television", CCIR Report 624-3 Volume XI, part 1, XVI Plenary Assembly, Dubrovnik, 1986; and PA0 2) "Video Waveform Drawing", EIA RS170A. References are made to the RS170A composite video signal drawing for NTSC signals. For NTSC signals, the subcarrier used for the determination for the color frame is a continuous sinewave with the same instantaneous phase as the burst. PA0 (1) Y.sub.D.sbsb.O =0.625 Y.sub.D.sbsb.I PA0 (2) I=0.625 [(1.031) CR+(-0.477) CB] PA0 (3) Q=0.625 [(0.699) CR+(0.730) CB] PA0 (a) low pass filter I PA0 (b) low pass filter Q
More details about this signal can be found in specification CCIR 601 incorporated herein by reference.
There is also a growing consensus in the electronics industry supporting a format for digital representation of a video composite signal.
The video encoding parameter values for the digital composite signal are defined in Table 2 below. More details about this signal can be found in the following two documents incorporated herein by reference:
TABLE 2 ______________________________________ Encoding Parameters for Digital Composite Signals ______________________________________ Coded Signal NTSC PAL (525 line system) (625 line system) Number of samples Total Active Total Active per line: 910 768 1135 948 Sample Structure Orthogonal Non-Orthogonal Sample Frequency 14.31818 MHz 17.734475 MHz (4 F.sub.sc) (4 F.sub.sc) Correspondence between signal levels and quantization levels: (in hexadecimal notation) 8 bit system Blanking level 3Ch 40h White level C8h D3h Sync level 04h 01h ______________________________________
It is clear from Tables 1 and 2 that the encoding parameters for the digital component and digital composite video signals are different, and therefore, video equipment operating in accordance with differing standards would be unable to exchange video data.
By way of further background, there are three levels of signals produced for color television. At the highest quality level, the video signal produced by the television camera has red (R), green (G), blue (B) signal components. At the next level, referred to as the component level, the video signals correspond to one luminance (Y) and two chroma signals, a signal designated CR and a signal designated CB. At the lowest level, the video signals are in the (United States) NTSC or (European) PAL format, which includes a luminance (Y) signal and two chroma signals, designated I (or U) and Q (or V). These are the digital NTSC (or PAL) components, where the chroma signals are modulated and added to the luminance signal to form the digital composite signal. As used herein, the term digital component signal refers to a digital representation of the color video signal at the component (Y, CR, CB) level, and the term digital composite signal refers to a digital representation of the color video signal at the composite (NTSC, PAL) level. It will be understood by those skilled in the art that the invention described and claimed herein is applicable for us in both the United States (NTSC) and European (PAL) color television systems, and references to the Y, CR, CB, U and V signals and PAL composite signal are for illustrative purposes only and should be understood to encompass the corresponding representations of the corresponding signals under the NTSC standards.
There is a need to convert digital video signals between the video component signal format and the video composite signal format. The video component signal is a better quality signal and can be manipulated more easily than the video composite signal. Thus, the digital component signal format is usually used by professionals for production work for doing such things as inserting computer graphics, resizing a picture, overlapping, matting and creating special effects. Once the production work is completed, the digital component signal format must be converted to the digital composite signal format since it is more closely related to the
or NTSC standards.
Conversion between the digital component signal format and the digital composite signal format requires several steps. One approach, as described in the prior art for conversion from digital component to digital composite is as follows:
1) Matrixing of digital component signal components to digital composite signal components in accordance with the following mathematical relationship (wherein D.sub.O indicates the output digital composite signal and D.sub.I is the input digital component signal):
2) Band limiting of the chroma components:
3) Modulation of the 3.58 MHz carriers by the chroma components and forming the composite signal: EQU NTSC COMPOSITE VIDEO=Y.sub.D.sbsb.O +I cos [2.pi.(3.58 MHz)t]+Q sin [2.pi.(3.58 MHz)t]
The system for producing the digital component signal format uses a clock frequency of 13.5 MHz. On the other hand, the system for producing the digital composite signal format uses a clock frequency of 14.318 MHz. Thus, in converting from one signal format to the other, the differences in clock frequencies must be taken into account. Due to the differences in clock frequency, some of the digital samples in one of the signals may be lost; therefore, interpolation techniques are required to prevent the loss of information. An interpolation method and apparatus are detailed in U.S. Pat. No. 5,057,911, assigned to the present assignee, the teachings of which are hereby incorporated by reference.
The chroma signal portion of the video signal in the digital component signal format has a bandwidth of 2.75 MHz, whereas the chroma signal portion of video signals in the digital composite signal format has a maximum bandwidth of 1.3 MHz. The chroma signal must be combined during the conversion process according to the mathematical relationship set forth above to produce the proper chroma signals for the digital composite format into which the digital component signal is to be converted. Differences in signal levels for each format must be taken into account by providing gain adjustments for the signals. Finally, in the conversion from digital component format to the digital composite signal format, the chroma signal must be modulated and the resulting composite signal corrected for DC offset.
Prior art systems required a relatively large amount of hardware because of the complexity of the conversion. It is therefore an objective of the present invention to provide a system for conversion between the digital component and digital composite signal formats which reduces the amount of hardware previously required.
It is another objective of the present invention to provide a relatively simple method for converting from digital component to digital composite signals.
It is yet another object of this invention to facilitate the communication of video information between two dissimilar digital video streams.
It is a further object of the invention to provide for an interface between two video equipment units having differing input and output characteristics.